This Week in Science

Science  12 Oct 2012:
Vol. 338, Issue 6104, pp. 169
  1. Vesta to the Core


    Vesta is one of the largest bodies in the main asteroid belt. Unlike most other asteroids, which are fragments of once larger bodies, Vesta is thought to have survived as a protoplanet since its formation at the beginning of the solar system (see the Perspective by Binzel, published online 20 September). Based on data obtained with the Gamma Ray and Neutron Detector aboard the Dawn spacecraft, Prettyman et al. (p. 242, published online 20 September) show that Vesta's reputed volatile-poor regolith contains substantial amounts of hydrogen delivered by carbonaceous chondrite impactors. Observations of pitted terrain on Vesta obtained by Dawn's Framing Camera and analyzed by Denevi et al. (p. 246, published online 20 September), provide evidence for degassing of volatiles and hence the presence of hydrated materials. Finally, paleomagnetic studies by Fu et al. (p. 238) on a meteorite originating from Vesta suggest that magnetic fields existed on the surface of the asteroid 3.7 billion years ago, supporting the past existence of a magnetic core dynamo.

  2. Distinguishing Right from Left

    In most vertebrates during embryonic development, rotational movement of the cilia within a structure in the embryo, known as the node, generates unidirectional flow required for future left-right asymmetry of the internal organs. The flow may transport a determinant molecule or provide mechanical force. However, it is not clear how the flow is sensed. Yoshiba et al. (p. 226, published online 13 September; see the Perspective by Norris and Grimes) show that nodal flow in mouse embryos is sensed by the cilia of perinodal cells located at the edge of the node, in a manner dependent on Pkd2, a Ca2+-permeable cation channel that has been implicated in polycystic kidney disease in humans.

  3. Bend to Straighten

    At low temperatures, the behavior of disordered solids, such as glasses, deviates from that of ordered crystals. The deviations may stem from the ability of some atomic entities to tunnel between two sites of almost identical energy, forming two low-energy states; such two-level systems (TLSs) are also thought to be a major contributor to the decoherence of superconducting qubits. Grabovskij et al. (p. 232) used mechanical strain to control the splitting between the energy levels of TLSs formed in the disordered barrier of the Josephson junction in a superconducting qubit. For some of the detected TLSs, the splitting exhibited the predicted minimum as a function of strain, verifying the TLS model of disordered solids.

  4. Rise and Subside

    Earth's surface tends to deform as magmatic fluids rise toward the surface from below, usually manifesting itself in uplift of continental crust. Fialko and Pearse (p. 250; see the Perspective by Brooks), however, show that subsidence, or widespread sinking, accompanies uplift when magma rises. Satellite measurements reveal that the massive Altipano-Puna magma body in the central Andes balloons upward, causing subsidence around the region of uplift, resembling a sombrero. Melting of surrounding rocks as the magma rises likely withdraws material and causes the subsidence.

  5. Embryonic Cell Sorting and Movement

    Differential cell adhesion has long been thought to drive cell sorting. Maître et al. (p. 253, published online 23 August) show that cell sorting in zebrafish gastrulation is triggered by differences in the ability of cells to modulate cortex tension at cell-cell contacts, thereby controlling contact expansion. Cell adhesion functions in this process by mechanically coupling the cortices of adhering cells at their contacts, allowing cortex tension to control contact expansion. In zebrafish epiboly the enveloping cell layer (EVL)—a surface epithelium formed at the animal pole of the gastrula—gradually spreads over the entire yolk cell to engulf it at the end of gastrulation. Behrndt et al. (p. 257) show that an actomyosin ring connected to the epithelial margin triggers EVL spreading both by contracting around its circumference and by generating a pulling force through resistance against retrograde actomyosin flow.

  6. Relatively Cold


    Temperature is essentially a measure of relative atomic or molecular motion. Low temperature does not necessarily imply a sample at absolute rest—what is important is for every member of the sample to be moving (or not moving) at the same velocity. Techniques for studying reactions under extreme cooling have nonetheless tended to focus on slowing down molecules. Henson et al. (p. 234) now demonstrate an alternative approach in which two beams of distinct gas-phase reagents are merged so as to continue forward with very little spread in their velocity. The interactions thus occur at millikelvin temperatures, revealing signatures of nonclassical dynamics such as oscillatory ionization probabilities with small shifts in energy.

  7. The Right Choice?

    So-called irrational decisions made by humans are popular fodder for “believe it or not” stories. But what's actually happening when we make choices that do not seem to be justifiable on purely economic or logical grounds? Presumably, we are not simply making errors; instead, our choices may reflect an internal bias that we are not aware of. Wimmer and Shohamy (p. 270) show how the hippocampus can instill an unconscious bias in valuations, whereby an object that is not highly valued on its own, increases in value when it becomes implicitly associated with a truly high-value object. As a consequence, we then end up preferring the associated object over a neutral object of equal objective value while not really knowing why.

  8. Feeling the Light

    There has been a link missing in our understanding of the biochemical signaling mechanism initiated when photons are absorbed by rhodopsin in the photoreceptor cells of Drosophila eyes. Photoisomerization of rhodopsin activates a heterotrimeric guanine nucleotide—binding protein which leads to activation of phospholipase C. But how phospholipase C activates the transient receptor potential (TRP) or the transient receptor potential–like (TRPL) ion channels that produce the rest of the cellular response has been unclear. Hardie and Franze (p. 260; see the Perspective by Liman) propose that there is a physical mechanism at work. Atomic force microscopy revealed light-induced contractions of photoreceptor cells. Consistent with a physical coupling mechanism, manipulations of the membrane altered responses of the photoreceptor cells to light. Thus, physical changes in the membrane appear to couple phospholipase C activity to the opening of mechanosensitive TRP and TRPL channels.

  9. Cheat Control

    In quorum-sensing induction, a Pseudomonas aeruginosa population growing on a single carbon source, such as casein, will reach a density where the levels of signaling molecules they collectively secrete triggers the cells to synthesize and secrete proteases to digest the casein. However, it is metabolically costly to secrete proteases, and the system is prone to mutant “cheats.” These cheats do not respond to quorum sensing and do not go to the cost of synthesizing protease, but they do profit from the breakdown products that allow all the cells—cheats and cooperators alike—to grow. Dandekar et al. (p. 264) found that quorum signaling–insensitive P. aeruginosa cheats could not synthesize nucleotide hydrolase and were thus unable to grow if casein was replaced by adenosine. This allowed cooperators to outgrow the cheats, leading to a stable equilibrium between cheats and cooperators. This principle of regulation may be applicable to other bacterial quorum-sensing systems and might be exploited in the development of drugs that disrupt bacterial cooperation.

  10. Ancient Genomics


    The Denisovans were archaic humans closely related to Neandertals, whose populations overlapped with the ancestors of modern-day humans. Using a single-stranded library preparation method, Meyer et al. (p. 222, published online 30 August) provide a detailed analysis of a high-quality Denisovan genome. The genomic sequence provides evidence for very low rates of heterozygosity in the Denisova, probably not because of recent inbreeding, but instead because of a small population size. The genome sequence also illuminates the relationships between humans and archaics, including Neandertals, and establishes a catalog of genetic changes within the human lineage.

  11. Mobile Phone “Hot Spots”

    An obstacle to developing effective national malaria control programs is a lack of understanding of human movements, which are an important component of disease transmission. As mobile phones have become increasingly ubiquitous, it is now possible to collect individual-level, longitudinal data on human movements on a massive scale. Wesolowski et al. (p. 267) analyzed mobile phone call data records representing the travel patterns of 15 million mobile phone owners in Kenya over the course of a year. This was combined with a detailed malaria risk map, to estimate malaria parasite movements across the country that could be caused by human movement. This information enabled detailed analysis of parasite sources and sinks between hundreds of local settlements. Estimates were compared with hospital data from Nairobi to show that local pockets of transmission likely occur around the periphery of Nairobi, accounting for locally acquired cases, contrary to the accepted idea that there is no transmission in the capital.